Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Flexography is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Furthermore, due to product competition, the market requirements on the printing quality of the images on the packaging can be very stringent.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of in order, a backing, or support layer; one or more unexposed photocurable layers; optionally a protective layer or slip film; and often a protective cover sheet.
The support sheet or backing layer lends support to the plate. The support sheet, or backing layer, can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyelefins, polyamides, and the like. One widely used support layer is a flexible film of polyethylene terephthalate.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. More than one photocurable layer may be used.
Photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of polymerizing, crosslinking or curing the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions. One commonly used source of actinic radiation is a mercury arc lamp, although other sources are generally known to those skilled in the art.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light, and the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate.
In a “digital” or “direct to plate” plate making process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is generally a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are then ablated by exposing the masking layer to laser radiation at a selected, wavelength and power of the laser. Examples of laser ablatable layers are disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter (i.e., thermal development). Thermal development processes work by processing photopolymer printing plates using heat; the differential melting temperature between cured and uncured photopolymer is used to develop the latent image.
The resulting surface, after development, has a relief pattern that reproduces the image to be printed and which typically includes both solid areas and patterned areas comprising a plurality of relief dots. After the relief image is developed, the relief image printing element may be mounted on a press and printing commenced.
The flexographic printing plate is mounted on a printing cylinder and the material to be printed, which is typically supplied as a continuous web, is placed between the printing roll and a backing roll. The flexographic printing plate is brought against the material with sufficient pressure to allow contact between the relief image on the plate and the material printed. In a typical process, an ink fountain pan supplies ink to a metering roll. A doctor blade may also be used to wipe off excess ink from the metering roll to assist in controlling the amount of ink that is on the metering roll.
In order to produce good images in flexographic printing, it is necessary that the ink be applied to the printed surface in a uniform and predictable manner. This in turn requires that the relief areas in the flexographic plate carry ink in a uniform layer and in predictable amounts.
One means of controlling the amount of ink applied to the printing plate uses a special ink metering roll, known as an “anilox” roll, which has on its surface a plurality of ink metering cells. These cells are small indentations arrayed in regular patterns of a predetermined frequency and of uniform depth and shape which are typically created by engraving the cylinder face using a mechanical process or by laser; the amount of ink delivered by the anilox roll is controlled by the screen size of the cells. During operation, ink is transferred from the ink well onto the anilox metering roll, filling the cells. The optional wiper blade wipes off excess ink from the roll surface leaving only the cells filled. The ink from the cells is then transferred onto the flexographic plate relief areas as the anilox roll and the flexographic plate rotate in contact with one another.
The images typically reproduced by flexographic plates almost always include both solid image areas and a variety of gray tone areas. “Solid areas” are defined as areas completely covered by ink having the highest density the ink can produce on a given material, while “gray areas” are defined as image areas where the appearance of the printed image is of a density intermediate to pure white (total absence of ink) and solid. Gray areas are produced by the half-toning process described herein, in which a plurality of relief surface areas per unit area of progressively larger surface area are used to produce the illusion of different density printing. These relief areas are commonly referred to as “halftone dots.”
In addition, flexographic printing is what is known as a “binary system,” meaning that it either prints or it does not. When relief areas contact the printed surface, one gets a substantially solid color area. To create a gray scale in flexographic printing, a process called “half-toning” is used, wherein gray tones are reproduced by printing a plurality of minute solid dots per unit area and varying either the frequency of the dots per unit area or the size of the dots per unit area or both.
In a flexographic plate, these halftone dots are relief areas having their surface at the top surface of the plate. The plate in the area surrounding the dot has been etched to a depth which except for the darkest areas reaches to a floor. The height of the halftone dot is the distance of the surface of the dot (and plate surface as well) to the floor, which can be referred to as the “halftone relief.” This relief decreases as the % dot coverage increases, and is sufficient to confine ink to the dot surface.
Halftone relief is controlled by a number of factors, including the etching process used to remove the material from between the dots. In a photopolymer flexographic printing plate the maximum relief depth is controlled by a back exposure of the plate which hardens the photopolymer to a desired depth and establishes a floor and thus a maximum relief depth.
In “classic” halftoning, amplitude modulated (AM) screening is used to produce halftone dots in regular repeating patterns of X-number of dots per linear inch. These patterns are identified by the percentage coverage of a given area by the dot surface area within the given areas as 1% dots, 5% dots, 95% dots, 98% dots, etc. A 98% dot means that 98% of a given area is occupied by the dot surface size. A 2% dot means that 2% of the same given area is occupied by the dot surface area therein.
In the alternative, typically referred to a “stochastic” halftoning, frequency modulated (FM) screening is used to increase the frequency of occurrence of the dots to produce higher and higher surface area coverage and the dot size is held constant. In addition, as described in U.S. Pat. No. 5,892,588 to Samworth, the subject matter of which is herein incorporated by reference in its entirety, a combination of the two techniques may be used to improve the visual appearance of the printed image.
The shape of the dots and the depth of the relief, among other factors, affect the quality of the printed image. It is very difficult to print small graphic elements such as fine dots, lines and even text using flexographic printing plates while maintaining open reverse text and shadows. In the lightest areas of the image (commonly referred to as highlights) the density of the image is represented by the total area of dots in a halftone screen representation of a continuous tone image. For AM screening, this involves shrinking a plurality of halftone dots located on a fixed periodic grid to a very small size, the density of the highlight being represented by the area of the dots. For Frequency Modulated (FM) screening, the size of the halftone dots is generally maintained at some fixed value, and the number of randomly or pseudo-randomly placed dots represent the density of the image. In both cases, it is necessary to print very small dot sizes to adequately represent the highlight areas.
It is also a known problem in flexographic printing, that solid areas (i.e., areas in the image where there are no half tone dots), appear to print with less saturation and somewhat less uniformity than halftone areas representing dark image areas. Thus an area with a dot coverage of 95% to 98% may appear darker than a solid area (100%), A problem in printing solid areas in flexography is uneven ink transfer over the full solid image area, which can cause a lack of density and a halo effect (i.e., a darker border) along the edges of the solid image area.
The level of color saturation achievable during flexographic printing is dependent upon many factors, prominent among which is the amount and uniformity of ink which can be applied to the print substrate, particularly in solid areas. This is commonly referred to as “Solid Ink Density” (SID). SID is sometimes higher at tone levels less than 100%, e.g., the optical print density achieved at the 97% tone level is slightly higher than that achieved at a 100% (solid) tone.
This observation has led to the development of a number of technologies for introducing fine reverse patterns into the solids of flexographic plates, expressly for the purpose of increasing the achievable SID. Notable examples include DigiCap (available from Kodak) and Groovey Screens (available from Esko-Graphics). DigiCap applies a user definable texture pattern to the surface of a flexographic printing plate to improve ink transfer and the appearance of solid areas, especially when printing on high hold-out substrates such as film or coated paper stocks. Groovy Screens, a hybrid screening technology, uses traditional AM screening throughout most of an image, but adds a line pattern (or “grooves”) into the dark, shadow areas and solids. The transition between the normal screen pattern and the line pattern is gradual, leading to a smooth gradation in print between the lower density of the non-groovy print (highlights and midtones) and the higher density (shadows) of the groovy print. Although somewhat effective, these techniques often require considerable experimentation and fine control to achieve consistent success, and can also have negative interactions (e.g., moiré) with the graphic images being printed.
Maintaining small dots on flexographic plates can be very difficult due to the nature of the platemaking process. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (the dot is not “held” on plate and/or on press). Alternatively, if the dot survives processing they are susceptible to damage on press. For example small dots often fold over and/or partially break off during printing causing either excess ink or no ink to be transferred.
Photocurable resin compositions typically cure through radical polymerization, upon exposure to actinic radiation. However, the curing reaction can be inhibited by molecular oxygen, which is typically dissolved in the resin compositions, because the oxygen functions as a radical scavenger. It is therefore desirable for the dissolved oxygen to be removed from the resin composition before image-wise exposure so that the photocurable resin composition can be more rapidly and uniformly cured and to improve the overall plate structure.
Thus, while various methods have been proposed for improving the quality of the printing plate, there remains a need in the art for additional improvements in the art that can provide a desirable result, especially in improving the achievable solid ink density of flexographic printing elements.